LNG (liquefied natural gas) generally refers to colorless, transparent cryogenic liquid converted from natural gas (predominantly methane) that is cooled to approximately −163° C. and condensed to 1/600th the volume.
As LNG emerges as an energy source, efficient transportation means have been sought in order to transport LNG from a supply site to a demand site in a large scale so as to utilize LNG as energy. Resulted in a part of this effort is LNG carriers, which can transport a large quantity of LNG by sea.
LNG carriers need to be furnished with a cargo tank that can keep and store cryogenically liquefied LNG, but such carriers require intricate and difficult conditions. That is, since LNG has vapor pressure that is higher than atmospheric pressure and boiling point of approximately −163° C., the cargo tank that stores LNG needs to be constructed with materials that can withstand very low temperature, for example, aluminum steel, stainless steel and 33% nickel steel, and designed in a unique insulation structure that can withstand thermal stress and thermal contraction and can be protected from heat leakage, in order to keep and store LNG safely.
Here, the structure of a cargo tank insulation of an LNG carrier is described below. FIG. 1 is a sectional view illustrating the conventional cargo tank structure of an LNG carrier. As illustrated in FIG. 1, a bottom insulation panel 10 is adhered and fixed by way of a fixing plate 10a to an internal face of a hull 1 of an LNG carrier by epoxy mastic 13 and a stud bolt 14. Arranged above the bottom insulation panel 10 is a top insulation panel 20, and a rigid triplex 22 is interposed between the bottom insulation panel 10 and the top insulation panel 20.
The rigid triplex 22, which is an insulation panel to which the bottom insulation panel 10 and the top insulation panel 20 are attached, is pre-manufactured in a shop and supplied into the cargo tank to constitute a secondary barrier of the cargo tank.
When the insulation panel, such as the bottom insulation panel 10 and the top insulation panel 20, is adhered to a cargo tank wall, a gap 40 is formed between the adjacent bottom insulation panels 10 so that a flat joint 18 made of a glass wool material can be inserted in the gap 40.
Then, a top bridge panel 28 is attached in between the top insulation panels 20 by adhering a supple triplex 26 over the rigid triplex 22, which is already attached, with epoxy glue 24 and then adhering the top bridge panel 28 over the supple triplex 26 with epoxy glue 24.
The top insulation panel 20 and an upper part of the top bridge panel 28 have a same planar surface, on which a corrugated membrane 30, in which a plurality of corrugations are formed, is attached as a primary barrier to complete the insulation structure of the cargo tank of an LNG carrier.
Although a continuity-guaranteed sealed barrier has been conventionally provided through the supple triplex 26 over the gap 40 between the bottom insulation panels 10, there has been a problem of a possible drop in sealing effect if an adhesive impregnated in the triplex has a high viscosity. Moreover, in case thermal load is repeatedly exerted on the composite material of the triplex, a crack can occur due to a difference in the coefficient of thermal expansion between internal reinforced fiber and resin, thereby possibly causing a gas leak.